This invention relates to an apparatus for raising or lowering personnel, equipment, and/or materials, and in particular an apparatus in which vertical motion is unaccompanied by any substantial horizontal motion.
Various designs employing multiple booms are known in the prior art. Some designs have employed gears, chains, or cables in the linkages which operatively connect the upper boom to the lower boom. Controlled by these linkages, elevation of the latter causes elevation of the former. Additionally, such linkages have been used to maintain the work station, which includes platforms, baskets, and buckets, in a level attitude. However, such mechanisms are complex, costly to manufacture and maintain, and require periodic adjustment by trained field personnel in order to remain properly operable.
Canadian Patent No. 1,016,084 issued to J. A. Merrick on Aug. 23, 1977 relates to an elevating device for raising platforms. The device includes first and second parallelogram linkages. In one embodiment, both the first and second parallelogram linkages are each raised and lowered by the operation of independent hydraulic cylinders. In another embodiment, Merrick replaces either of the hydraulic cylinders in the first-mentioned embodiment with a mechanical link. Thus, for instance, the patent illustrates an apparatus in which the first and second parallelogram linkages are operatively interconnected by a rigid compression link.
The movement of each parallelogram linkage by an independent hydraulic cylinder suffers at least two drawbacks. First, it is often advantageous for the work station to be movable vertically while unaccompanied by substantial horizontal movement, thus allowing accurate placement of the apparatus in the work area before elevation without the need to adjust the apparatus ground position thereafter. Where the boom assemblies are of equal length, and movable in opposite directions, this may only be achieved by simultaneously extending or retracting both boom assemblies at the same speed. In the device of the Merrick patent, careful and skillful synchronization of the hydraulic cylinders by the operator is required to accomplish this result. Alternatively, systems which automatically synchronize the hydraulic cylinders are known, however these relatively complex systems require maintainence by skilled personnel. Second, the incorporation into a design of multiple hydraulic cylinders and the relatively complex control means necessary to synchronize them undesirably increases manufacturing expense.
U.S. Pat. No. 4,019,604 issued to L. D. Benson on Apr. 26, 1977, relates to apparatus for elevating a platform, the apparatus described include a pair of lift boom assemblies which are interconnected such that the platform can be raised and lowered in a level attitude. Each boom assembly includes a pair of arms which are parallel to each other. The two lift boom assemblies are interconnected at a so-called floating frame and are operatively connected by a rigid tension link. This structure has drawbacks since the quality of welds used in tension is critical to the integrity of the tension link and especial care must be exerted in its manufacture.
The Merrick patent describes the second parallelogram linkage of one embodiment as having a "skew" or offset on each arm consisting of two bends of equal degree. The stated purpose of this skew is to prevent interference between the first linkage and the second linkage when the two are moved past each other to the retracted or stored position. Such designs are desirable in that the apparatus is foldable into a compact form for ease of stowage and transport within areas of limited height and width.
Similarly, this offset design feature is incorporated in one embodiment of the Benson patent. The lower lift boom assembly pivots downwardly from an upstanding post member supported by the mobile frame when the apparatus is being retracted. Both arms of the upper lift boom assembly are formed with an offset or "zig-zag" such that the two lift boom assemblies can be stowed side-by-side when the apparatus is in the retracted position without interfering with each other.
However, the use of an offset in both arms of a boom assembly or parallelogram linkage is less than desirable. Not only are the designs complex, they are also difficult and therefore costly to manufacture. For example, the Benson apparatus requires a plurality of welds or fasteners to maintain the structural integrity of the boom arms. Similarly, bending the arms in accordance with the Merrick patent requires costly bending processes in order to achieve the proper amount of offset.
Alternatively, the offsetting of one boom assembly from the other also has been achieved using a double floating frame. However, such a design is not advantageous because such frames are significantly heavier than their single frame counterparts, thus requiring the utilization of larger and costlier lifting means as well as enhanced means for stablizing the extended apparatus.
A difficulty encountered in utilizing a compression link in conjunction with a single floating frame to interconnect the first and second boom assemblies is that, when the compression link is pivotally connected to the upper arm or main boom of the second boom assembly, the axis of the link must necessarily intersect the axis of the lower arm of the second boom assembly. The Merrick patent describes one solution to this problem. Merrick shows a lower arm made of two parallel bars held in a spaced-apart relationship by spacers, thereby allowing the compression link to operate therebetween. However, such a design is less than satisfactory because, since it requires several parts to be fabricated, aligned, and fastened together to form the lower arm, the manufacture is costly.